Diamond by vapor phase synthesis is known as a way to obtain polycrystalline diamond with a relatively large surface area less expensively than natural diamond or manmade monocrystalline diamond obtained under ultra-high pressure, and such diamond is utilized in optical component applications and as heat sinks for electronic components and tools. Known film formation methods include microwave plasma CVD, hot filament CVD, DC arc jet plasma CVD, and so forth. Diamond obtained by these methods usually exhibits electrically insulating property, but can be rendered conductive by doping impurities during the film formation. Such conductive diamond has for some time been the subject of research and development in semiconductor and electronic component applications, particularly in the growth of monocrystalline diamond in the vapor phase, but imparting conductivity to polycrystalline diamond produced by vapor phase synthesis has been attracting particular attention in recent years because of its potential for use as water treatment electrodes.
In order to treat large quantities of water, diamond electrodes used for water treatment are used in a state in which a large electric current is sent to large electrodes. Therefore, to improve the efficiency of the treatment, it is important for the electrical resistance of the diamond layer, which is the outermost surface of the electrode, to be low.
The most widely known method for producing polycrystalline conductive diamond for electrodes in water treatment is to dope boron during film formation by microwave plasma CVD or hot filament CVD.
Many different methods are known to dope boron during film formation, of which the followings are specific examples.
Japanese Patent Publication 2001-147211 discloses an invention relating to a method for measuring stably and at high sensitivity the amount of uric acid in a liquid measured by using a diamond thin-film electrode that has been anodized. Here, in the manufacture of a diamond thin film by microwave plasma CVD, boron oxide (B2O3) is dissolved in a mixture of acetone and methanol, this solution is introduced into an apparatus using H2 gas as a carrier gas, and a film is formed.
Japanese Patent Publication H9-13188 described, in relation to a diamond electrode in which at least part of the electrode is composed of a semiconductor diamond film, wherein diborane (B2H6) diluted with hydrogen is used as a raw material gas to form a diamond film by microwave plasma CVD.
Japanese Patent Publication 2000-313982 relates to an electrode produced by forming a diamond layer on a substrate, wherein boron is doped in the diamond by using trimethyl borate (B(OCH3)3) as the boron source in the formation of the diamond layer by hot wire CVD (hot filament CVD), and the boron content is from 10 to 10,000 ppm, and preferably 10 to 2000 ppm, and even more preferably 5 to 1000 ppm.
The publication “Preprints of the 26th Committee of Electrolytic Science Technology, Committee of Soda Technology (Denkai Gijutsu Touronkai—Soda Kougyou Gijutsu Touronkai Youshishu), pp. 1-4” states that when an electrolysis test is performed using a conductive diamond film formed on a silicon substrate or niobium substrate, a diamond electrode may not have adequate durability due to separation of the diamond film or corrosive wear of the substrate, depending on the solution or electrolysis conditions.
The important points for a diamond electrode are that a large surface area can be coated with diamond, and that the electrical resistance of a diamond layer is reduced for the sake of electrical efficiency of the electrode, that is, that boron is doped in a large quantity. Also, in the formation of a conductive diamond film as an electrode on a substrate with a large surface area, the film needs to have enough physical and chemical strength and adhesion to be able to withstand use in harsh environments of high current density and high potential, corrosive environments such as electrolytic corrosion, and separation caused by stress generated between the substrate and the conductive diamond.
In a method in which trimethyl borate is doped with a filament CVD, it has been indicated that doping a large quantity of boron makes the potential window smaller, so the doped amount cannot be raised. The same thing has been mentioned in Patent Document 2, which states that the doping a large quantity of boron adversely affects diamond film quality and prevents the properties inherent in diamond from being obtained. Thus, existing methods have had a number of problems in terms of stably producing low-resistance conductive diamond by doping a large quantity of boron to a diamond electrode with a large surface area, and making the substrate thereof sufficiently durable.
Patent Document 1: Japanese Patent Publication 2001-147211
Patent Document 2: Japanese Patent Publication H9-13188
Patent Document 3: Japanese Patent Publication 2000-313982
Non-Patent Document 1: Preprints of the 26th Committee of Electrolytic Science Technology, Committee of Soda Technology (Denkai Gijutsu Touronkai—Soda Kougyou Gijutsu Touronkai Youshishu)